1. Field of the Invention
This invention relates to highly branched, liquid polymers of ethylene and/or .alpha.-olefins, e.g., propylene, which are prepared in the presence of an aluminum-based catalyst system, optionally containing a transition metal component. The polymers prepared in accordance with this invention have unique properties and are useful, for example, as thickeners and as lubricants and/or lubricant additives. An advantage of the present invention is that it enables the synthesis of highly branched ethylene and .alpha.-olefin polymers without the need for transition metal catalyst components, thereby avoiding disposal problems typically associated with the use of transition metal-based catalysts.
2. State of the Art
Polymerization of ethylene in the presence of transition metal-based catalysts usually results in the formation of solid, linear polymers. As such, they are not suitable as soft materials or lubricants for most applications.
For a hydrocarbon polymer having a molecular weight more than about 350 to remain liquid, the polymer typically must contain significant branching. An example of such moderately branched hydrocarbon polymers can be found in PCT Int. Appl. WO96/23010 (1996). In that PCT application, it is disclosed that polymers having a moderate degree of branching can be synthesized by using palladium and nickel catalysts incorporating very bulky chelating .alpha.-diimine bidentate ligands. The PCT application discloses, for example, polyolefins having about 80 to about 150 branches per 1000 methylene groups, wherein for every 100 branches that are methyl branches, there are about 30 to about 90 ethyl branches, about 4 to about 20 propyl branches, about 15 to about 50 butyl branches, about 3 to about 15 amyl branches, and about 30 to about 140 hexyl or longer branches. The olefin polymers described in the PCT application are said to be useful as elastomers, molding resins, and in adhesives. Polymers containing monomer units derived other than from olefins are also disclosed in the PCT application; and polymers which contain olefin and olefinic ester monomer units, particularly copolymers of ethylene and methyl methacrylate and/or other acrylic esters, are said to be useful as viscosity modifiers for lubricating oils.
Oily hyperbranched polymers derived from ethylene, propylene, butene and/or other .alpha.-olefins, and a method for their synthesis, are disclosed in WO98/33823 (1998), which is assigned to the assignee of the present application. WO98/33823 discloses polymers having non-regular microstructures characterized by a ratio (R) of methyl hydrogens centered around 0.85 ppm on the 1H-NMR spectra of the polymers relative to total aliphatic hydrogens of from about 0.40 to about 0.65 for polymers derived from ethylene or butene, and a ratio (R) of from greater than 0.50 to about 0.65 for polymers derived from propylene. However, the catalyst system employed to synthesize the hyperbranched polymers described in WO98/33823 is transition metal-based and differs significantly from the aluminum-based catalyst system contemplated in the present invention.
Other disclosures that describe nickel-based catalyst systems for ethylene polymerization include, for example, Macromol. Rapid Commun. 1997, 18, 795, which discloses using a catalyst system comprising nickel .alpha.-diimine compounds, and Macromol. Rapid Commun. 1997, 18, 1017, which discloses using a catalyst system comprising a [.eta.3-methylallyl-nickel-dad]PF.sub.6 complex that is active in the presence of an organoaluminum compound. The polymers that are formed using the catalysts systems that are described in the above references are significantly less branched that the polymers prepared in accordance with the present invention.
Other catalyst systems that are useful for the synthesis of moderately branched polymers from ethylene have been disclosed. See, for example, J. Polym. Sci. Chem. Ed. 1984, 22, 3027; U.S. Pat. No. 4,735,931 (1988); PCT Int. Appl. WO94/07930 (1994); EP 0 727 446 A1 (1996); PCT Int. Appl. WO96/07680 (1996); PCT Int. Appl. WO97/48735 (1997); EP 0 250 999 B1 (1997); U.S. Pat. No. 5,686,542 (1997); and J. Am. Chem. Soc. 1998, 120, 1082. The catalyst systems that are described in the above references typically are based on transition metal Ziegler-type or metallocene catalysts and result in polymers that are essentially linear or only moderately branched.
More highly branched polymers of ethylene (having a degree of branching comparable with that of the polymers prepared in accordance with the present invention) are described in J Am. Chem. Soc. 1998, 120, 1932. The polymers that are disclosed in that reference are prepared in the presence of a Group 10 transition metal-based catalyst, e.g., a Ni- or Pd-based catalyst. Accordingly, the process disclosed in that reference is typical in the sense that it must deal with the disposal of transition metal components.
There have been few reports of transition metal free catalyst systems that are useful for the synthesis of polymers from ethylene and/or .alpha.-olefins. One such report (Makromol. Chem., 1992, 193, 1283) discloses an aluminum based catalyst system that produces polymers that are "largely linear" and are characterized by molecular weights of at least 300,000. In a more recent report (J. Am. Chem. Soc., 1997, 119, 8125), an aluminum catalyst containing nitrogen-based ligands was disclosed for the synthesis for hydrocarbon polymers. However, the polymers that were disclosed in that report were not significantly branched. Similarly, U.S. Pat. No. 5,777,120 discloses cationic aluminum amidinate compounds that are useful Ziegler Natta-like olefin polymerization catalysts, and WO98/40421 discloses cationic complexes comprising a Group 13 element and certain ligands that behave like Ziegler-Natta polymerization catalysts in the absence of any transition metal. The polymers that are prepared using the catalysts of both U.S. Pat. No. 5,777,120 and WO98/40421 are essentially linear polymers of ethylene.
While great strides have been made in the search for new and improved ethylene and .alpha.-olefin polymerization catalysts, there remains a need for catalyst systems that are not based primarily on transition metals, that require only commercially available aluminum-based components, that require no ligand substitution, and that, nonetheless, are capable of efficiently converting ethylene and/or olefin monomer(s) to highly branched, liquid polymers under otherwise conventional polymerization reaction conditions.